Gas sensors are used in a variety of applications such as to detect noxious or explosive gases, or to measure the quantity of a particular gaseous component in a mixture of gases. For example, gas sensors are used to monitor oxygen in certain air-fired shale retorting processes and coal gasification processes. In combustion control applications, information regarding the relative concentrations of combustion gas components can be used to generate a feedback signal to regulate the combustion process. This control provides a means for maximizing efficient fuel usage and for managing exhaust emissions.
One type of known gas sensor uses a solid electrolyte material which exhibits ion-specific conduction to "sense" the quantity of a particular gas in a gas mixture. Porous metal electrodes are attached to the opposite faces of the solid electrolyte to form a galvanic cell. By exposing one face of the cell to a reference gas of known concentration and the opposite face to an unknown concentration of the same gas, the cell generates a galvanic potential which can be used to determine the unknown concentration. The galvanic potential is produced by the gas concentration gradient across the electrolyte body. If there is no concentration gradient, the cell voltage is zero. The voltage can be related to the gas partial pressure differential at the two electrolyte faces by the Nernst equation: E=AT ln[P.sub.1 /P.sub.2 ] where E is the galvanic voltage, T is the absolute temperature of the gas, P.sub.1 /P.sub.2 is the ratio of partial pressures of the gas, and A=R/4F, where R is the universal gas constant and F is the Faraday constant.
Gas sensors have been devised which comprise both a solid-electrolyte, galvanic cell and a device incorporated in the sensor which generates an internal gas reference. These gas sensors generate their own internal reference through a second solid electrolyte cell having porous metal electrodes which electrochemically "pumps" gas into and out of a fixed-volume, hermetically sealed cavity within the sensor. An external power source is used to apply a potential across the solid electrolyte body. Gas molecules are ionized at the interface of the gas, the negative electrode and the electrolyte by acquiring electrons flowing through the negative electrode. These ions then move through the solid electrolyte body by ionic conduction. At the positive electrode, gas ions give up electrons and recombine into gas molecules. By reversing the polarity of the circuit, gas can be transported in either direction. By pumping gas into and out of the sealed cavity through the solid electrolyte, while simultaneously sensing the partial pressure differential between the cavity gas and the external gas with a solid electrolyte galvanic cell, these internal-reference, solid-electrolyte gas sensors measure the concentration of a particular gas in a gas mixture environment.
One particular application for gas sensors of the type described above is in the automotive industry for use in analyzing automobile exhaust gases. It is known that the partial pressure of oxygen in automobile exhaust gas has a direct relationship to engine air-to-fuel ratio. By measuring the oxygen content of the exhaust gas, a feedback signal can be generated which allows the air-to-fuel ratio to be altered in order to achieve optimum combustion conditions. This control over engine combustion facilitates economical fuel usage and provides a means for regulating exhaust emissions. In order for a gas sensor to generate a signal which can be used in an automotive feedback system, the sensor must be accurate and capable of completing its analysis very rapidly. For example, an automotive exhaust gas sensor should have a response time of less than 0.1 second at a minimum temperature of 300.degree. C. and a minimum oxygen concentration of about eight percent. The sensor must be airtight to prevent the leakage of oxygen into or out of the sealed chamber and be free of any source of current leakage which can be caused by electronic conduction in the electrolyte body. Both types of leakage would produce false sensor readings. The sensor must also have sufficient structural integrity to absorb shock associated with use in an automobile as well as have the ability to withstand thermal expansion of its materials over a temperature range of at least -40.degree. C. to 800.degree. C. Finally, such a sensor must be suited to be mass-produced.
Internal-reference, solid-electrolyte gas sensors have in the past been constructed from discrete components. A typical sensor having this construction is disclosed in U.S. Pat. No. 3,907,657 to Heijne et al. Two solid-electrolyte discs are coated on their opposite faces with porous metal electrodes and then bonded to opposite ends of a ceramic or metal, e.g. platinum, cylinder to form a sealed cavity. The cavity serves as the reference chamber into and out of which a selected gas is pumped through one electrolyte disc by a reversible constant current. In the ceramic cylinder-type of sensor, a passage through the cylinder wall must be made in order to connect leads to the inner electrodes. These lead pathways must then be hermetically sealed. In the platinum type of sensor, the platinum cylinder body provides a path for electrical contact with the inner electrodes. Leads are attached respectively to the internal and external electrodes by spot welding. Gas sensors fabricated from discrete components in the manner described above are extremely fragile and are difficult to accurately replicate.
Since sensor response time is proportional to the cavity volume, attempts have been made to reduce the size of conventional discrete gas sensors. However, conventional sensor designs and fabrication techniques prevent significant sensor miniaturization due to inherent limitations in the materials used and assembly difficulties. Prior attempts in producing a miniaturized gas sensor have resulted in a discrete component assembly having an internal volume of approximately 0.16 mm.sup.3 with pump electrolyte cell wall thicknesses of approximately 0.75 mm. The response time of such a sensor is at best approximately 0.5 second in a mixture of gases containing about ten percent oxygen. This response time is much too slow for any meaningful automotive exhaust feedback application.
The rate of gas transport through the pump cell can be increased or decreased by increasing or decreasing the cell voltage. However, the amount of gas which can pass through the cell at a given temperature and voltage is limited by the resistance of the electrolyte to ionic conduction. Also, an increase in electrolyte cell thickness is accompanied by a proportional increase in its resistance to ion conduction. Although thinning the cell increases the ionic conductivity, it decreases the structural integrity of the electrolyte which must at least be sufficient to withstand slight pressure differentials across its body. It is also important to observe that when the voltage applied to the pump cell is increased above a certain threshold value, the electrolyte undergoes electrochemical reduction which can lead to failure of the material. Therefore, the pump voltage must not exceed the reduction potential of the specific electrolyte material used in the sensor.